As an efficient coding and compression tool, Context Adaptive Binary Arithmetic Coding (CABAC) is used extensively in AVC (or JVT), as described in Draft ITU-T Recommendation and Final Draft International Standard of Joint Video Specification (ITU-T Rec. H.264| ISO/IEC 14496-10 AVC), March 2003. When enabled, all syntax elements below slice header layer are coded using CABAC in AVC.
The following are operations involved in decoding CABAC-coded streams:
Initialization—the decoding engine is first initialized using already decoded properties about a slice. Range division variables used in the decoding engine are initialized to known values and the context model variables are also initialized.
Binarization—Each syntax element to be decoded is expressed in a variable length code at the encoder side and the process of converting a fixed-length code to a variable length code is called binarization. The purposes of employing binarization are to assign a string of bits to syntax elements with more than two possible values, and to assign shorter codes to the more probable values for the syntax element. At the decoder side, a de-binarization process is applied so that the original fixed-length syntax element can be recovered.
Symbol Decoding—The basic element that CABAC is dealing with is a binary bit value of ‘1’ or ‘0’ (referred to as a bin). A binarized syntax element would have a string of such binary bits and each such bit is called a symbol in CABAC. Each symbol of a syntax element is decoded individually with a probability model associated with the symbol. In CABAC, a symbol may have several models (or contexts) associated with it and the model selection is based on adjacent macroblock properties. A table-based approach is employed in CABAC to decode a symbol that, in principle, emulates the range subdivision algorithm as developed in “Arithmetic Coding for Data Compression”, Communications of the ACM, Vol. 30, No. 6, June 1987, by Ian H Witten, et. al. and “Arithmetic Coding Revisited”, ACM Transactions on Information Systems, Vol. 16, No. 3, July 1998, pp. 256-294, each of which are incorporated herein by reference, and that uses symbol probability models as variables.
Model Update—After a symbol is decoded, the probability model (or context model) will be updated based on the decoded value of the symbol. So next time when the same symbol is decoded again using the same context model, the probability values will be different. Through this, an adaptive model updating is being achieved.
Reference management—The context selection for certain binary bits (bins) in a syntax element are based on the values of previously decoded syntax elements in geometrically adjacent left and top macroblocks. When AFF (adaptive frame-field) is enabled for a bit stream, every macroblock pair can be either frame or field coded. There is a set of rules that need to be followed based on the properties of the current macroblock pair and the adjacent ones in order to derive the reference value associated with a geometrically adjacent block/macroblock. These references are in turn used to calculate the context associated with the bin-to-be decoded.
There are several difficulties with CABAC. Among them are:
Each binarized symbol bit (or bin) is decoded one at a time and involves invoking the range subdivision algorithm.
Each bin involves selection of a context and updating the context at the end of the decoding of the bin.
Various bins of a syntax element may use different context selection criteria.
Different syntax elements use different binarization schemes.
Initialization of all the context-models takes a lot of processing because of the large number of context models involved.
Certain bins for a syntax element have multiple contexts associated with them. The context to be used for those bins is calculated based on previously decoded syntax elements in geometrically adjacent macroblocks and macroblock pairs. The storage structure of the decoded syntax elements for different macroblocks and macroblock pairs and selection of the appropriate reference and calculation of the context to be used are both inherently complex.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.